Nowadays mercury-cadmium telluride is considered highly suitable for the fabrication of optoelectronic devices operating at high wavelengths. More particularly, because of its low energy gap between valence and conduction bands, it is suited to the fabrication of photodetectors for infrared radiation with wavelengths in the 8 and 14 um. In fact, the energy necessary to produce electron-hole pairs in the semiconductor is of only 0.1 eV. Optoelectronic devices can thus be fabricated for detecting low-energy radiations, such as, for instance, those emitted by objects at room temperature.
Mono- or bidimensional arrays, forming the photosensitive elements of high-performance imaging systems can be made from devices of this kind. These systems can supply thermal images useful in various applications, by instance illness diagnosis, terrestrial pollution mapping from on board satellies, sighting of objects or people under poor visibility conditions, etc.
High performance electronic devices are fabricated using a basic semiconductor material available as single crystal. In fact only in this case does the material present physical properties which are constant and well known at each point of its volume; that is why the performances of the devices obtained therefrom may be optimized. Besides, the single crystal obtained must present a lattice periodicity as perfect as possible, in order to avoid undesired reductions in the quantatative efficiency of photodetection.
Such characteristics can be better obtained by epitaxial growth techniques, which also allow the formation of wide surfaces necessary to the fabrication of photodetector arrays. Liquid and vapor phase fabrication techniques are well known. The former are nowadays the most-widely used.
Liquid-phase epitaxial growth consists of the deposition on a single-crystal substrate of alloys with compositions depending on those of suitable growth solutions. Starting from solutions wherein melted tellurium acts as solvent and cadmium and mercury as solutes, single-crystal layers of the ternary mercury cadmium telluride compound are deposited on the binary cadmium telluride compound, generally used as substrate. The deposition takes place by bringing the substrate into contact with the slightly supersaturated solution.
The crystal growth solution has been till now prepared in a cycle different from that in which epitaxial growth takes place. The preparation method consisted of sealing in a quartz ampoule convenient quantities of tellurium, cadmium and mercury, of homogenizing them at a temperature higher than 500.degree. C. for some tens of hours, and of rapidly quenching them to preserve the homogeneous composition of the liquid.
Unluckily, this growing technique, as well as vapor phase technique, is highly complicated by the strong tendency of mercury to evaporate. This disadvantage is favored by the necessity of causing the growth process to evolve at temperatures higher than the melt temperature of solvent tellurium, equal to about 500.degree. C. Mercury evaporation causes composition variations during growth thermal cycles both in the single crystal solid and in the solution it is generated by (in liquid-phase techniques).
To overcome this disadvantage different methods have been suggested to protect the solution composition. A method described in "Semiconductors and Semimetals", Academic Press, Vol. 18, 1981, pages 70-84, suggests the insertion in a sealed ampoule of both the solution and the substrate on which the deposition is effected by bringing into contact the solution and solid. Howver this method has been abandoned owing to the difficulties encountered in controlling growth phases inside the closed ampoule and to the impossibility of implementing structures with different composition layers.
According to another method, described by T. C. Harman in the papers issued in Journal of Electronic Materials, Vol. 9, No. 6, pages 945-961 and Vol. 10, No. 6, pages 1069-1085 the solution and substrate are placed in a reactor with hydrogen atmosphere at atmospheric pressure and the solution composition is preserved by means of a mercury source inside the reactor itself. The source consists of a mercury bath maintained at a lower temperature than that of the growth solution, but such as to produce a partial mercury vapour pressure equal to that of solution equilibrium. That is obtained by placing the reactor in a furnace with two zones at different temperatures.
This method, according to the literature, requires the solution to be prepared in a phase preceding the growth phase. This is a disadvantage due to possible contamination of the solution during handling and certainly the duration of the process is longer.